A cutterhead is disclosed in published German patent application 2,855,990 wherein the flexible cutting filament is automatically readjusted by a predetermined length by means of a pivotally mounted latching lever which releases the spool. The cutting filament is usually a plastic filament which wears down during the cutting operation and becomes shorter. The automatic readjustment of the filament takes place in that the rotational speed of the cutterhead and therefore its centrifugal force acting on the latching lever increases as the projecting segment of the cutting filament becomes ever shorter. This centrifugal force moves the latching lever against the biasing force of a spring into an outer release position wherein the latching lever releases a latch stop of the spool so that the spool can rotate relative to the cutterhead and thereby unwind a segment of filament.
As the filament unwinds, the resistance becomes greater and the rotational speed becomes less so that the spring force acting on the latching lever again returns the latching lever to its initial position in which a latch cam of the latching lever engages in a corresponding latching recess and latches the spool.
A disadvantage of this configuration is that there is no precise return of the latching lever to its position wherein the spool is latched and the return takes place essentially through a spring force in dependence upon centrifugal force.
It is a further disadvantage of this configuration that the latching cams provided in the recesses lie in the peripheral region of the spool hub. Accordingly, the latching cams are on a relatively small peripheral region and therefore little space is provided for the configuration of the latching lever and especially for the latching cams. This leads to high contact pressures in the latched position which, in turn, lead to an increased wear and a premature exhaustion of the cutterhead.
U.S. Pat. No. 4,607,431 discloses a cutterhead wherein a supply spool for the cutting filament is rotatably mounted in the housing of the cutterhead and is likewise provided with a latching arrangement. This latching arrangement includes a one-arm latching lever pivotally journalled in the housing and a switching arm which coacts with switching stops of a switching disc. The switching stops are provided over the periphery of the spool and the latching lever coacts with the switching disc in such a manner that when the cutting filament is worn, the latching lever transfers out of its latching position with respect to the switching stops of the spool and into an outer position in which the latch on the spool is removed. A basic difference however is that with this arrangement, the filament pulled from the spool is guided around the pivotable latching lever such that a filament redirecting structure is formed directly on the latching lever. The filament redirecting structure defines a point of application for the centripetal force since the rotating cutting filament is subjected to centrifugal force which has a varying magnitude in dependence upon the rotational speed. The centripetal force is directed opposite to the centrifugal force acting on the latching lever and tends to move the latching lever inwardly into its latched position. Accordingly, in the cutterhead described in U.S. Pat. No. 4,607,431, the cutterhead only responds to the mass of the free end of the filament which, in turn, is a direct function of its length. On the other hand, the cutterhead disclosed in published German patent application 2,855,990 includes a latching lever which is not subjected to the centrifugal force of the cutting filament. A disadvantage of this configuration is that an automatic readjustment is not assured for an intense wear of the cutting filament. Accordingly, the operator must make the readjustment manually especially when there is a great amount of wear and the exposed filament segment is very short. This readjustment is made for example by controlling the throttle of the engine of the vegetation cutter. In this configuration, the cutting filament is not biased with adequately large return forces which are intended to transfer the latching lever from the release position into the latching position. This is true even when the one-arm latching lever is guided in a slot in the manner of a rocker arm with the rocker arm having center segments and straight line guide path segments extending therefrom between adjacent latch positions.
U.S. Pat. No. 4,651,421 discloses a cutterhead having a two-arm latching lever which is not biased by the cutting filament; that is, the latching lever is not subjected to the centrifugal force of the cutting filament as in the cutterhead described in published German patent application 2,855,990. In this configuration, the latching lever coacts with individual stop cams arranged on the spool so that the latching lever biased by the force of the return spring reaches an outer release position for the spool when the centrifugal force working thereon is greater than the returning spring force.
In the embodiment of U.S. Pat. No. 4,651,421, two latching levers are arranged symmetrical to each other and are in meshing engagement with the spool via an intermediate ring in such a manner that the switching operations take place synchronously. As in the configuration first described herein, the disadvantage is here also present that there is no precise return of the latching levers notwithstanding the synchronous action and manual intervention is required by the operator.